This invention relates to a frequency-shift keying (FSK) receiver and, in particular, to a FSK receiver of double superheterodyne type for demodulating an FSK modulated signal using a local oscillation signal generated by a phase-locked oscillator as a first local oscillation signal.
There are two types in FSK receivers. A first type is an FSK receiver including, as a local oscillator, a phase-locked oscillator having a quartz crystal resonator. Such an FSK receiver can deal with a lot of line or channel frequencies using the quartz crystal resonator. This is because the phase-locked oscillator of the first type oscillates a local oscillation signal having a variable local frequency. A second type is an FSK receiver including, as a local oscillator, a quartz crystal oscillator which oscillates a local oscillation signal having a fixed local frequency.
The FSK receiver may be a radio paging receiver. The radio paging receiver is used in various service areas assigned with the line frequencies, respectively, which are different from each other. It will be assumed that a called person carrying a radio paging receiver transfers from a service area assigned with a line frequency from another service area assigned with another line frequency. In this event, it is impossible to call the called person who carries the radio paging receiver of the second type. On the other hand, it is possible to indisputably call the called person who carries the radio paging receiver of the first type.
In general, the radio paging receiver is activated by a battery. In order to save the battery, the radio paging receiver is operable in an intermittent operation in the manner well known in the art. In spite of battery saving, the radio paging receiver of the first type consumes much current in comparison with that of the second type. This is because the phase-locked oscillator carries out a phase-locked loop (PLL) control. As a result, the radio paging receiver of the first type shortens battery's life.